The present invention relates to a communications antenna a structure and cellular base stations and in particular, but not necessarily restricted thereto, relates to micro-cellular and pico-cellular basestations and antennas therefor.
In addition to public cellular radio systems, cellular communications systems are being developed for use in a local area, e.g. in a factory or an office building to provide a wireless communications service. The base station employs antennas to transmit and receive radio frequency signals over, for example an outdoor micro-cell with a 200 m radius, or an indoor pico-cell with a 30-50 m radius. In such a system, communication takes place over a radio interface between user handsets and one or more base stations. Each base station is provided with an antenna structure whereby to communicate with user handsets in its particular service area. Such micro-cellular and pico-cellular basestations may also be employed in a public cellular system to cover xe2x80x9cdeadxe2x80x9d spots such as those which may be generated within a shopping mall or similar, or otherwise supplement the coverage provided by a wide area coverage cellular basestation.
Typically the base station is in a xe2x80x9cclutteredxe2x80x9d environment such that energy received from a given handset arrives at a basestation via many paths. The path can be direct (line of sight) or indirect due to reflections and diffractions from local scatterers (buildings, furniture, ground, ceiling, etc.). The basestation is said to be in a multipath environment. The incoming multipath field vectors can add constructively or destructively, and this depends upon the relative amplitude and phase of the different components. Since the phase of each component will vary independently as a handset is moved, the received signal at the basestation varies considerably in magnitude, and this effect is known as multipath fading.
One way of overcoming multipath fading is to provide two channels for the received signal such that they are independent of each other i.e. the signals are uncorrelatedxe2x80x94when one channel is in a fade, the other channel is typically unfaded. Consequently, the base station selects the channel with the strongest signal to overcome fading whereby to provide a reliable communications link. This is a form of diversity reception. One way of providing diversity reception is to use two antenna elements, which is known as antenna diversity. When the basestation switches between the elements, instead of combining the elements, then this is known as switched antenna diversity. Accordingly, it is a requirement of the antenna structure is to provide polarisation and/or space diversity and also to provide a substantially uniform beam pattern so that there are no xe2x80x98deadxe2x80x99 spots in the area served by the base station and so that the orientation of a user handset has substantially no effect on the call quality.
A further requirement of a base station antenna structure is to provide a structure which is suitable for both wall and ceiling mount and to provide a uniform coverage with reasonable gain (xcx9c0 dBi) sufficient to service a significantly large area. It will be appreciated that, as base stations are relatively costly to manufacture and maintain, there is a significant cost advantage in providing effective service areas so as to minimise the number of base stations required for a particular installation. It has been found difficult to provide this uniformity of coverage in a compact antenna structure.
For modern telecommunications applications, apart from the electrical performance of the antenna other factors need to be taken into account, such as size, weight, cost and ease of construction of the antenna. With the increasing deployment of cellular radio, an increasing number of base stations which communicate with mobile handsets are required. Such antennas are required to be both inexpensive and easy to produce. A further requirement is that the antenna structures be of light weight yet of sufficient strength to be placed on the top of support poles, rooftops and similar places and maintain long term performance over environmental extremes.
The conventional approach to the problem of achieving diversity is the provision of a simple dipole structure which has been found adequate for many applications. However, at the frequencies involved (825-895 MHz and 1850-1990 MHz) the dimensions of the conventional dipole may be inconveniently large. Urban planning authorities are now demanding that base stations that are exposed to public view be enclosed in a relatively unobtrusive plastics housing which is generally too small to accommodate both a conventional dipole and the electronic equipment required for operation of the base station. In close proximity the dipole will interact with the base unit and create a non-uniform coverage pattern. A number of small antenna structures have been described, but these do not provide the desired combination of both coverage and diversity for successful employment as base station antenna.
GB-B-2291271 and U.S. Pat. No. 5,757,333 to Northem Telecom Ltd. provide a communications antenna structure, e.g. for a cellular radio base station which comprises first and second bent folded monopole planar antenna elements mounted on a ground plane and disposed generally perpendicular thereto. The antenna elements are mutually spaced from each other and are disposed with their respective planes at an angle to each other whereby to provide both polarisation diversity and space diversity of the antenna structure.
The present invention seeks to provide an improved cellular radio base station. The present invention further seeks to provide a cellular radio base station operable in horizontal and vertical modes of polarisation in first and second mutually perpendicular orientations. The present invention also seeks to provide substantially orthogonal polarisation over several full plane cuts and, a substantially null free coverage in the transmit combined pattern provide a method of operating
In accordance with a first aspect of the invention there is provided a communications antenna structure comprising:
a body having an axis with a first surface parallel to said axis and a second surface orthogonal to said axis;
wherein the first and second antennas each have an axis about which a mode of electrical field vector polarisation can be generated and wherein the first and second antennas are attached to the first and second surfaces respectively which antennas are positioned on the body whereby the axes of polarisation are orthogonal with respect to each other such that:
in a first orientation, with said body axis vertical, the first antenna operates in a horizontal mode of electrical field vector polarisation and the second antenna operates in a vertical mode of electrical field vector polarisation; and,
in a second orientation, with said body axis horizontal, the first antenna operates in a vertical mode of electrical field vector polarisation and the second antenna operates in a horizontal mode of electrical field vector polarisation.
Preferably, the antennas are planar inverted F antennas (PIFA). The first and second planar surfaces are preferably parallel to the planar portions of the first and second PIFAs. The first and second surfaces can be planar.
Conveniently, the body is a generally rectangular box. Conveniently the body encloses the electrical control circuitry associated with the receiver and transmitters for the antennas and the outside surface has heat sink fins to assist in the maintenance of a suitable operating temperature for the electronics.
In accordance with another aspect of the invention, there is provided a communications antenna structure, comprising:
an electrical control circuitry;
a first and a second planar inverted F antennas; and,
an earthed shielding structure having an axis, which is arranged to enclose said electrical control circuitry;
wherein the first and second antennas are positioned such that:
in a first orientation, vertical relative to said axis, the first antenna operates in a horizontal mode of electrical field vector polarisation and the second antenna operates in a vertical mode of electrical field vector polarisation; and,
in a second orientation, horizontal relative to said axis, the first antenna operates in a vertical mode of electrical field vector polarisation and the second antenna operates in a horizontal mode of electrical field vector polarisation.
In accordance with another aspect of the invention, there is provided a communications antenna structure, comprising:
first and second planar inverted F antennas; and,
a structure having an axis;
wherein the first and second antennas each have an axis about which a mode of electrical field vector polarisation can be generated and which axes of polarisation are orthogonal with respect to each other such that:
in a first orientation, vertical relative to said axis, the first antenna operates in a horizontal mode of electrical field vector polarisation and the second antenna operates in a vertical mode of electrical field vector polarisation; and,
in a second orientation, horizontal relative to said axis, the first antenna operates in a vertical mode of electrical field vector polarisation and the second antenna operates in a horizontal mode of electrical field vector polarisation.
In accordance with another aspect of the invention, there is provided a radio communications base station arrangement, comprising:
first and second linearly polarised antennas; and,
a structure having an axis;
wherein the first and second antennas each have an axis about which a mode of electrical field vector polarisation can be generated and which antennas are positioned whereby the axes of polarisation are orthogonal with respect to each other such that:
in a first orientation, upright relative to said axis, the first antenna operates in a horizontal mode of electrical field vector polarisation and the second antenna operates in a vertical mode of electrical field vector polarisation; and,
in a second orientation, horizontal relative to said axis, the first antenna operates in a vertical mode of electrical field vector polarisation and the second antenna operates in a horizontal mode of electrical field vector polarisation.
Preferably, the antennas are planar inverted F antennas. The structure to which the antennas are mounted comprises a body with a first surface parallel to said axis and a second surface orthogonal to said axis and wherein the first and second antennas are attached to the first and second surfaces respectively. Conveniently, the body is generally cuboid. Conveniently the body encloses the electrical control circuitry associated with the receiver and transmitters for the antennas and the outside surface has heat sink fins to assist in the maintenance of a suitable operating temperature for the electronics.
In order to reduce costs it is preferred to have a common physical dimensions to both 1900 MHz and 800 MHz basestations, with the mechanics designed to allow attachment of either an 800 MHz or 1900 MHz antenna.
In accordance with a still further aspect of the invention, there is provided a method of operating a communications antenna structure, comprising: a body having an axis with a first planar surface parallel to said axis and a second planar surface orthogonal to said axis; wherein the first and second antennas each have an axis about which a mode of electrical field vector polarisation can be generated and wherein the first and second antennas are attached to the first and second surfaces respectively which antennas are positioned on the body whereby the axes of polarisation are orthogonal with respect to each other, the method comprising the steps of:
in a first orientation, with said body axis parallel relative to the vertical, feeding rf signals to the first antenna whereby the antenna operates in a horizontal mode of electrical field vector polarisation and feeding rf signals to the second antenna whereby the antenna operates in a vertical mode of electrical field vector polarisation; and,
in a second orientation, with said body axis parallel relative to the horizontal, feeding rf signals to the first antenna whereby the antenna operates in a vertical mode of electrical field vector polarisation and feeding rf signals to the second antenna whereby the antenna operates in, a horizontal mode of electrical field vector polarisation.
In accordance with a still further aspect of the invention, there is provided a method of operating a communications antenna structure, comprising: first and second planar inverted F antennas; and, a structure having an axis; wherein the first and second antennas each have an axis about which a mode of electrical field vector polarisation can be generated and which antennas are positioned whereby the axes of polarisation are orthogonal with respect to each other, the method comprising the steps of:
in a first orientation of the base, said axis being vertical, feeding rf signals to the first antenna whereby the antenna operates in a horizontal mode of electrical field vector polarisation and feeding rf signals to the second antenna whereby the antenna operates in a vertical mode of electrical field vector polarisation; and,
in a second orientation of the base, said axis being horizontal, feeding rf signals to the first antenna whereby the antenna operates in a vertical mode of electrical field vector polarisation and feeding rf signals to the second antenna whereby the antenna operates in a horizontal mode of electrical field vector polarisation.
In accordance with a still further aspect of the invention, there is provided a method of operating a radio communications base station arrangement, comprising: first and second linearly polarised antennas; and,
a structure having an axis; wherein the first and second antennas each have an axis about which a mode of electrical field vector polarisation can be generated and which antennas are positioned whereby the axes of polarisation are orthogonal with respect to each other, the method comprising the steps of:
in a first orientation of the base, said axis being vertical, feeding rf signals to the first antenna whereby the antenna operates in a horizontal mode of electrical field vector polarisation and feeding rf signals to the second antenna whereby the antenna operates in a vertical mode of electrical field vector polarisation; and,
in a second orientation of the base, said axis being horizontal, feeding rf signals to the first antenna whereby the antenna operates in a vertical mode of electrical field vector polarisation and feeding rf signals to the second antenna whereby the antenna operates in a horizontal mode of electrical field vector polarisation.
In a yet still further aspect of the invention there is provided a cellular communications network incorporating any of such communications antenna structures.